71:1 



;i = i,i . 10-7, 



which agrees very well with the theoretical one 1,13. lO^". 



We must observe, however, that we cannot assign to our measu- 

 rements a greater precision than of lO"/»- 



It seems to us tiiat within these limits the theoretical conclusions 

 have been fairly contirmed by our observations.. 



The experiments have been carried out in the "Physikalisch-Tech- 

 nische Reichsanstalt". We want to express our thanks for the a[)pa- 

 ratus kindly placed at our disposition. 



Physics. — '''On a possible injiii£nce of the FRKs^v,L-coe/'ficieiit on 

 soldi' phenomena". By Prof. P. Zeeman. 



(Communicated in the meeting of September 25, 1915). 



We shall prove here, that the presence of the term of 



^ ' n cD. 



LoRËNTZ in the expression for the Fresnel coefficient (cf. also my 



paper Vol. 18, p. 398 of tiiese Proceedings) may give rise to a 



change in the propagalion of lightwaves if in a moving, refracting 



medium a change of velocity occurs. I suppose the medium to have 



everywhere the same density and to be flowing with a velocity v 



parallel to the axis of X in a system of coordinates that is at rest with 



respect lo the observer. In the direction of the Z axis a velocity 



gradient exists in such a way, that the velocity decreases with the 



distance to the A' axis and becomes zero at the distance z -- A. If 



now the incident lightbeam (with a plane wave front) is parallel to 



the axis of A', the parts of the wave fronts which are near this 



axis will be more carried with the medium than those at a greater 



distance. The wave front will thus be rotated. 



If ihe velocity decreases linearly in the direction of the Z axis 



the wavefront will remain plane. In a time / the angle of rolalion, 



(supposed to be small) will be <(=——, where e is the Fkksnel 



coefficient and where v and L have the above mentioned meaning. 



More in general we may consider an element of the wave front 



dv I' 



and then wi-ite - for — . Moreover t may be expressed as a func- 

 dz A ' ' 



lion of the velocity of light and the path through which the rays 



have travelled, so that we find 



d dv 



« = 7-y (1) 



c/fi ac 



46 



Proceedings Royal Acad. Arasterdam. Vol. XVIII. 



